1. Field of the Invention
The present invention relates to a method of crystallizing a semiconductor thin film, and a laser irradiation apparatus which is used for the method. Also, it relates to a thin film transistor and a display device, for example, LCD or organic EL display as are fabricated by utilizing the method and the apparatus.
2. Description of the Related Art
Crystallizing annealing which employs laser light has been developed as a part of an expedient by which a process for manufacturing a thin film transistor is turned into a low temperature process. This consists in that a non-monocrystalline semiconductor thin film of amorphous silicon, polycrystal silicon of comparatively small grain diameters, or the like formed on an insulating substrate, is irradiated with laser light to be locally heated, whereupon the semiconductor thin film is converted (crystallized) into a polycrystal of comparatively large grain diameters in the cooling process thereof. The thin film transistor is integrally formed using the crystallized semiconductor thin film as an active layer (channel region). Owing to the adoption of such crystallizing annealing, it is permitted to establish a low temperature process for a thin-film semiconductor device, and to use an inexpensive glass substrate, unlike an expensive quartz substrate of excellent heat resistance.
In the crystallizing annealing, it is common practice that the pulses of laser light in the shape of a line are intermittently projected while being overlapped in a scanning direction. The semiconductor thin film can be crystallized comparatively uniformly by projecting the laser light overlappingly. The crystallizing annealing which employs the laser light in the linear shape (line beam) is schematically illustrated in FIG. 11. The laser light 50 shaped into the line extending in the Y-direction of an insulating substrate 1 made of glass or the like is projected from the front surface side of the insulating substrate 1 formed with a semiconductor thin film beforehand. On this occasion, the insulating substrate 1 is moved in an X-direction (scanning direction) relatively to the irradiation region thereof. Here, the irradiation is done while the line beam 50 emitted from the light source of an excimer laser is being intermittently moved in overlapping fashion. More specifically, the insulating substrate 1 is scanned through a stage member in the X-direction relatively to the line beam 50. The stage member is moved at a pitch smaller than the widthwise dimension of the line beam 50 by one shot, so that the whole substrate 1 can be irradiated with the line beam 50, thereby to carry out the crystallizing annealing.
Laser light is sequentially outputted as pulses from a laser light source. The intensities (energy densities per unit area) of the individual pulses are not always constant, but they fluctuate in excess of xc2x115 [%] even with an up-to-date laser light source. Therefore, in a case where the laser light has been projected by overlapping the pulses repeatedly, local dispersion comes out in the diameters of the crystal grains of a crystallized semiconductor thin film in accordance with dispersion in the intensities of the individual pulses. This appears as dispersion in the characteristics of thin film transistors which are integrally formed on an insulating substrate. In a case where a display device, such as liquid crystal panel, has been fabricated using such an insulating substrate, the characteristics dispersion appears as non-uniformity in an image quality or as pixel defects.
FIG. 12 is a schematic plan view illustrating an example of a region of irradiation with a line beam. The irradiation region is in, for example, an elongate shape having longer latera of 200 [mm] and shorter latera of 400 [xcexcm], and it scans in the direction of the shorter latera. Irradiation regions adjacent to each other overlap at their parts of, for example, 95 [%]. Accordingly, the line beam having the shown irradiation region is moved stepwise at intervals of 20 [xcexcm]. When note is taken of one point on a substrate, the line beam passes 20 times at the steps of 20 [xcexcm], and the point is irradiated with laser light 20 times in total.
FIG. 13 is a graph schematically showing the sectional intensity distribution of the line beam along line Xxe2x80x94X indicated in FIG. 12. In general, the sectional intensity distribution of a line beam in the shorter axial direction thereof is in the shape of a rectangle. When the line beam scans at the steps of 20 [xcexcm], a certain point on an insulating substrate is intermittently irradiated with laser light 20 times. Thus, a semiconductor thin film corresponding to the point repeats melting based on the laser irradiation and solidification based on cooling, 20 times, and crystal grains enlarge meantime. In actuality, however, dispersion is involved in the intensities of individual laser beams as stated before. Accordingly, when one point is noted, it is not irradiated with energy being always at the same level, repeatedly 20 times, but it is struck by energy having a dispersion of about xc2x115 [%]. In general, the crystal grains enlarges more as the laser light intensity is higher, but they turn into microcrystals contrariwise when a critical intensity is exceeded. Accordingly, when an abrupt upward fluctuation in the energy exists during the repeated pulse irradiation, the crystal grains might turn into the microcrystals on the contrary. Especially in the case of noting one place, when the abrupt upward fluctuation of the energy occurs at the final step among the 20 times of repeated irradiating steps, the crystal state of the place ends in a microcrystalline one left intact. Conversely, when the line beam of high energy is abruptly projected at the initial step among the 20 times of repeated irradiating steps, hydrogen might ablate on the occasion of the melting of the semiconductor thin film of amorphous silicon which has contained the hydrogen in large amounts at the stage of forming the film. When the ablation occurs, the semiconductor thin film itself changes in quality, and no normal crystal grains can be obtained even by thereafter irradiating the thin film with the line beam repeatedly.
In order to solve the problems of the prior art as stated above, means to be explained below have been adopted. The present invention consists in a method of crystallizing a semiconductor thin film, having the shaping step of shaping laser light emitted from a laser light source, thereby to define a laser beam which has a predetermined intensity distribution in a predetermined irradiation region; and the irradiating step of repeatedly irradiating the semiconductor thin film formed over a substrate beforehand, with the laser beam while scanning the film so that irradiation regions may be overlapped; characterized in that said shaping step shapes said laser beam so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region. Preferably, said shaping step shapes said laser beam so that an intensity at said front end of said irradiation region may become lower in a range within 30 [%], as compared with an intensity of the peak. Also, said shaping step shapes said laser beam so that an intensity at the rear end of said irradiation region may become lower in a range exceeding 5 [%], as compared with the intensity of the peak. Besides, said shaping step shapes said laser beam so that the intensity of the peak may become lower in a range exceeding 10 [%], as compared with a critical intensity incapable of the crystallization. By way of example, said shaping step shapes said laser beam so that the sectional intensity distribution may become a triangle. Alternatively, said shaping step shapes said laser beam so that the sectional intensity distribution may become a parabola. In one aspect, said shaping step shapes said laser beam so that said irradiation region may become an elongate shape; and said irradiating step moves said irradiation region relatively to the substrate in a direction orthogonal to longer latera of the elongate shape so that longer latus parts of said elongate shape may be overlapped.
The present invention further comprehends a method of manufacturing a thin-film semiconductor device as utilizes the above method of crystallizing a semiconductor thin film. More specifically, the present invention consists in a method of manufacturing a thin-film semiconductor device, having the film forming step of forming a semiconductor thin film over a substrate; the shaping step of shaping laser light emitted from a laser light source, thereby to define a laser beam which has a predetermined intensity distribution in a predetermined irradiation region; the irradiating step of repeatedly irradiating the semiconductor thin film formed over the substrate, with the laser beam while scanning the film so that irradiation regions may be overlapped, thereby to crystallize said semiconductor thin film; and the working step of forming a thin film transistor by utilizing the crystallized semiconductor thin film for element regions; characterized in that said shaping step shapes said laser beam so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region.
Besides, the present invention comprehends a laser irradiation apparatus which is applied to the above method of crystallizing a semiconductor thin film. More specifically, the present invention consists in a laser irradiation apparatus which, in order to irradiate a semiconductor thin film formed over a substrate beforehand, with a laser beam, thereby to crystallize the semiconductor thin film, is furnished with a laser light source for emitting laser light; shaping means for shaping the laser light, thereby to define a laser beam having a predetermined intensity distribution in a predetermined irradiation region; and irradiation means for repeatedly irradiating the semiconductor thin film formed over the substrate beforehand, with the laser beam while scanning the film so that irradiation regions may be overlapped; characterized in that said shaping means shapes said laser beam so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region.
In addition, the present invention comprehends a thin film transistor which is fabricated by utilizing the method of crystallizing a semiconductor thin film and the laser irradiation apparatus as stated above. More specifically, the present invention consists in a thin film transistor having a multilayer construction which includes a semiconductor thin film, a gate insulating film stacked on one surface of the semiconductor thin film, and a gate electrode stacked on the semiconductor thin film through the gate insulating film, characterized in that said semiconductor thin film has been crystallized by shaping laser light emitted from a laser light source, thereby to define a laser beam which has a predetermined intensity distribution in a predetermined irradiation region, and repeatedly irradiating said semiconductor thin film with the shaped laser beam while scanning said film so that irradiation regions may be overlapped; and that said semiconductor thin film has been crystallized by the irradiation thereof with said laser beam especially shaped so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region.
Further, the present invention comprehends a display device of active matrix type as includes the above thin film transistors. More specifically, the present invention consists in a display device having a pair of substrates joined to each other through a predetermined gap, and an electrooptic substance which is held in the gap, wherein one of the substrates is formed with a counter electrode, while the other substrate is formed with a pixel electrode, and a thin film transistor for driving the pixel electrode, and wherein the thin film transistor is formed of a semiconductor thin film, and a gate electrode which is stacked on one surface of the semiconductor thin film through a gate insulating film, characterized in that said semiconductor thin film has been crystallized by shaping laser light emitted from a laser light source, thereby to define a laser beam which has a predetermined intensity distribution in a predetermined irradiation region, and repeatedly irradiating said semiconductor thin film with the shaped laser beam while scanning said film so that irradiation regions may be overlapped; and that said semiconductor thin film has been crystallized by the irradiation thereof with said laser beam especially shaped so that a sectional intensity distribution of said laser beam in the irradiation region as taken in a direction of the scanning may be convex, and that a peak of the intensity distribution may lie at a position which is between a front end and a rear end of said irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of said irradiation region.
According to the present invention, the laser beam is shaped so that the sectional intensity distribution of the laser beam in the irradiation region as taken in the scanning direction may be convex. Especially, the laser beam is shaped so that the peak of the convex sectional intensity distribution may lie at the position which is between the front end and rear end of the irradiation region in relation to the scanning direction and which is nearer to the front end with respect to the middle of the irradiation region. Accordingly, even in a case where the intensity of the laser beam has abruptly fluctuated upwards just at the position of the peak, and where the critical intensity has been exceeded to microcrystallize the semiconductor thin film once, the thin film has room for being thereafter repeatedly irradiated with the laser beam, and hence, the crystal state of the thin film is restorable. More specifically, owing to the peak brought nearer to the front end with respect to the middle, even when the thin film has been once microcrystallized due to the abrupt dispersion, it is restored into a polycrystal state again by the succeeding irradiation. Incidentally, the intensity at the front end part is suppressed low, so that even when the abrupt upward fluctuation of the intensity has occurred here, the temperature of the front end part does not rise extraordinarily, ablation being preventable. Besides, the energy intensity on the rear end side is suppressed somewhat lower as compared with the peak. Accordingly, even when the abrupt upward fluctuation of the energy has occurred here, it is not apprehended that the critical intensity will be easily exceeded, and that the microcrystallization of the thin film not being restorable will arise. In addition, even if the microcrystallization has arisen at the part of the peak, it can be restored by the succeeding laser beam irradiation at the somewhat lower intensity on the rear end side.